Work version concept (#2569)

Adds nano::work_version which is passed to all work_generate and work_validate methods (except for tests, ensured with an assert).

The functional changes are quite minimal, as every caller defaults to the only work version available (work_1):

- RPC work_generate, work_validate and block_create can receive an optional "work_version"
- The work websocket outputs the work version

These changes constitute the pipeline necessary for conditional work validation.